The term "photorefraction" has been used to describe the phenomenon where the refractive index of a material is altered upon exposure to light. Initially, the phenomenon was observed in a certain restricted class of crystalline materials that were photoconductive and exhibited large polar effects. An example of such a material is LiNbO.sub.3. There is also a version of this type of "photorefractive" behavior that can be obtained in organic polymeric materials. In these polymeric materials, the various photosensitive agents are added to the polymer in a guest-host format.
More recently, a new class of materials have been reported to exhibit significant "photorefractive" behavior. These materials are glasses in the family xSiO.sub.2 --(1-x)GeO.sub.2. The origin of the effect in these materials is totally different from that of the ferroelectric crystals mentioned above. An excimer laser (193 nm and 248 nm) induces a refractive index change in these materials, and the refractive index change stems from large absorption changes originating from "defects" in the glass structure. Because the effect was originally discovered in a single mode optical fiber and, since the major application of the induced index change has been to fabricate phase gratings in the fiber, this "photorefractive" behavior is often referred to as "fiber Bragg gratings" in the technical literature. The effect has been reported to have been extended to other binary SiO.sub.2 compositions such as P.sub.2 O.sub.5, SnO, and Ce.sub.2 O.sub.3. By far the largest induced refractive index change has been in the SiO.sub.2 --GeO.sub.2 system where values as large as 0.001 have been reported. The size of the refractive index effect may be increased in the SiO.sub.2 --GeO.sub.2 system by impregnating the glass with molecular hydrogen before exposure.
Tin-phosphorous oxyfluoride glasses are known and are disclosed in U.S. Pat. Nos. 4,314,031 and 4,379,070, which are relied upon and incorporated herein by reference. U.S. Pat. No. 4,314,031 discloses that tin-phosphorous oxyfluoride glasses desirably have a very low glass transition temperature, frequently below 100.degree. C., yet still exhibit excellent resistance to attack by moisture at elevated temperatures. U.S. Pat. No. 4,379,070 discloses the use of tin-phosphorous oxyfluoride glasses as a matrix material for the support of photosensitive and electric-field-responsive polycyclic aromatic hydrocarbon compounds. Neither of these patents, however, discloses or suggests that tin-phosphorous oxyfluoride glasses exhibit a "photorefractive" effect or the use of tin-phosphorous oxyfluoride glass as a photorefractive material.
It would also be useful to provide a material that exhibits a "photorefractive" effect that could be doped with a variety of materials for altering the optical properties of the devices made from the material, including inorganic and organic dopants.